Speed limit edition and usage with no value fails

[flomonster]

What happened?

When we create a speed limit with no default value 0 is given to editoast.
Core behavior is weird having a speed limit of 0 (it uses the rolling stock speed limit).

What did you expect to happen?

• Front should provide a null speed limit when not provided (allowed by the model) => provide a undefined/null speed limit when user provide empty field #6428
• Editoast should expect speed greater than 0 => fix speed limit restriction #6436
• Core should expect speed greater than 0 => fix speed limit restriction #6436
• We could provide a migration to transform all 0 speed to null => fix speed limit restriction #6436

How can we reproduce it (as minimally and precisely as possible)?

1. Use this infra
2. Create a train (left to right)
3. Its MRSP should be 80km/h

What operating system, browser and environment are you using?

• Browser: Firefox v109
• OS: Nixos v22.11
• Env: Local

OSRD version (top right corner Account button > Informations)

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